This invention relates to microwave transistor testing using automatic microwave impedance tuners in order to synthesize reflection factors (or impedances) at the input and output of said transistors.
Impedance tuners at RF and microwave frequencies are based, in most cases, on the slide-screw principle (FIG. 1). Such tuners comprise a low loss parallel plate transmission line (slabline, (1)) and metallic probes (slugs, (3)), which are inserted (4) into the slabline (1) and create adjustable reflection factors when approached to the central conductor (2) of said slabline (1). Capacitive coupling between said probes (slugs) and central conductor is the main cause for reflection of the electromagnetic waves travelling across the slabline. Said probes (3) are also movable in horizontal direction (5), parallel to the central conductor (2) of said slabline and allow changing also the phase of said reflection factor at RF frequencies. The area (73) on a Smith chart (72) which can be reached by the reflection factor of said impedance tuners (FIG. 4a) is called tuning range. The Smith chart is a polar representation of electrical impedances Z=R+jX, using a reflection factor T=(Z−Zo)/(Z+Zo), where Zo is the characteristic impedance of the transmission media, in our case 50 Ohms. The tuning range is important if transistors with very low or very high internal impedance need to be tested. The characteristic impedance of said Smith chart being 50 Ohms a tuner capable of creating reflection factors of 0.9 can reach impedances between a minimum of 2.6 Ohms and a maximum of 950 Ohms.
The electrical field distribution inside a slide screw tuner is shown simplified in FIG. 2a). A capacitive coupling (8) exists between the central conductor (9) and ground, which in this case is the slabline itself (13). As the probe (6) moves up and down (7) said capacitive coupling decreases and increases correspondingly. FIG. 2b) shows an electrical equivalent of said capacitive coupling, represented by a capacitor (11) between the center conductor (10) and ground (12). Said capacitor (11) can move horizontally (14) as well.
Reducing the distance (8) between probe and central conductor (9) increases the capacitance (11) and the amplitude of the reflection factor. Moving the probe across the axis of the slabline (14, parallel to the central conductor) changes the phase of the reflection factor.
This type of tuner (FIGS. 1 and 2) has a typical frequency response of the reflection factor as shown in FIG. 3; here trace (65) shows the maximum reflection obtainable when a short probe is used. Trace (66) shows said maximum reflection when a long probe is used. Short and long terms are used here to describe the size of said probes in direction of the central conductor of said slabline (76, FIG. 1). Said slabline is described here as a parallel plate airline. Trace (67) shows the reflection factor of said slide screw tuner when both probes are withdrawn from the slabline. FIG. 3 also shows three vertical traces marked Fo, 2Fo and 3Fo. Said traces mark the fundamental, second and third harmonic of the frequency (Fo) at which the tuner is used. In this case the short probe is used (trace 65), which creates sufficient reflection at Fo. As can be seen there is an important reflection at all said frequencies (Fo to 3Fo). Because only the reflection factor at Fo is controlled by the system software, however, the reflection factors at 2Fo and 3Fo are uncontrollable. This is a major disadvantage of such wideband impedance tuners, using parallel plate airlines (or slablines) [2]. The long probe does not create high reflection at 2Fo and 3Fo, but it cannot be used in this case, since it does not create enough reflection at Fo either. The tuning range inside a Smith chart (72) at the fundamental (73) and harmonic (74, 75) frequencies of a tuner using probes as in FIGS. 1 and 2 is shown in FIGS. 4a), b) and c).
A practical way of avoiding this phenomenon of uncontrollable harmonic tuning, beyond the method of using additional harmonic rejection tuners [2], is to use resonant probes. Resonant probes can be non-contacting [3] or contacting the central conductor [4]. Contacting resonant probes have been used before in harmonic rejection tuners [4]. However the probes in [4] do contact the central conductor, not the ground plane of the airline, create a high reflection at harmonic frequencies and do not allow adjustment of the amplitude of the reflection factor. This invention describes resonant probes for avoiding this side effect.